how to place a UI

where and how do you center or place a user interface. Seems like it would be under the Main Window Event Handlers, OPEN.

A user interface is the presentation of the application or mechanism to the person (user) who is destined to utilize the application or mechanism. Things like windows, menus, buttons, text fields, lists, etc. are items in, and therefore define, the user interface. A user interface, of itself, has no concept of being “placed” and is “centered” if it is at peace with the universe.

Do you mean where does the Window show up when its opened ?
There’s a property called Placement in the inspector for certain defaults like “Main Screen” etc